1. Field of the Invention
The present invention relates to a novel method for fabricating a fiber Bragg grating, which can increase the accuracy to fabricate a long and complex fiber grating structure. Especially, a fiber Bragg grating having a complex variance in refractive index and arbitrary phase shift is a very important optical communication element.
2. Description of the Related Art
Recently, several methods for fabricating a long and complex fiber grating structure, such as moving-fiber-scanning-beam method and sequential writing method, are continuously proposed. In these methods, a He-Ne laser interferometer is used to monitor the fiber position during the execution of UV-writing. However, the accumulative position reading errors due to the drift of the interference fringe of the interferometer and the inaccurate grating period estimation have caused significant influences on the fabrication of a long fiber Bragg grating (FBG) structure.
For example, Taiwanese Patent No. 434,431 discloses a method for writing arbitrary refractive index disturbances into a waveguide, which writes a movable waveguide directly by using beam and in a single-point way. The above-mentioned method varies refractive index along the waveguide simply by controlling the moving speed of the waveguide. However, the method is suitable only for a long-period fiber grating, not for a short-period fiber Bragg grating.
Furthermore, U.S. Pat. Nos. 6,834,977 and 6,813,079 disclose that a fiber grating is written into an optical fiber by sequential exposure to UV beam section by section through a phase mask, and an interferometer is used to monitor the position of each positioning point of a translation stage. However, the method for monitoring a written position of the optical fiber by using an interferometer causes the problem of the above-mentioned accumulative errors, and the period of a writing beam needs to be precisely calibrated in advance.
Moreover, U.S. Pat. No. 5,945,261 uses a principle in which luminescence is generated by exposing an optical fiber to UV beam. In the Patent, a small grating section is previously exposed, the intensity of the luminescence is detected for the use of feedback, and then a connected grating being long and having no discontinuous phases is formed. However, this method cannot have a phase shift arbitrarily inserted. U.S. Pat. Nos. 6,753,118 and 6,801,689 both have the same disadvantage caused by a method in which a grating is written by repeated exposures section by section with a feedback compensation, and a modified amount is calculated by a grating spectral response obtained after writing. However, this method is not easy to be monitored in time, and is suitable for grating modification after writing. U.S. Pat. No. 5,830,622 discloses that UV beam is additionally exposed to a specific position so as to adjust refractive index, leading to an additional phase shift. However, a second scan is needed, more time is taken, and a wanted phase shift cannot be easily obtained section by section.
M. J. Coke, et al. issues a paper entitled “Moving Fiber/Phase Mask-Scanning Beam Technique for Enhanced Flexibility in Producing Fiber Gratings with Uniform Phase Mask,” Electronics Letters, Vol.31, No. 17, 1488 (1995) 1490, in which a grating is written into an optical fiber by moving the optical fiber or a phase mask. However, the length of a fiber grating fabricated by this method is limited to the length of the phase mask, and thus, the sharpness of a written pattern is affected by the limitation of moving speed.
Adel Asseh, Heleg Storoy, et al. issues a paper entitled “Writing Technique for Long Fiber Bragg Grating with Complex Reflectivity Profile,” Journal of Lightwave Technology, Vol. 15, No. 8, 1419 (1997) 1423, in which a grating is written by repeated exposures section by section, and an interferometer is used to monitor the position of each positioning point of a translation stage. However, the writing beam is a pulse UV-light laser, and can introduce the problem of extra noise.
Ingemar Petermann et al. issues a paper entitled “Fabrication of Advanced Fiber Bragg Gratings by Use of Sequential Writing with a Continuous-Wave Ultraviolet Laser Source,” Applied Optics, Vol. 41, No. 6, 1051 (2002) 1056, in which a grating is written by repeated exposures section by section, and an interferometer is used to monitor the position of each positioning point of a translation stage. Although the writing beam is a continuous UV beam, the method of monitoring the written position of an optical fiber by using an interferometer can cause the problem of accumulative errors, and the period of a writing beam needs to be precisely calibrated in advance.